The field of this invention is turbochargers of the type used to provide pressurized combustion air to an internal combustion engine. Particulary, this invention relates to a turbocharger including a housing journaling an elongate shaft for rotation with a turbine and a compressor. The turbine and compressor are spaced apart at opposite ends of the shaft, and the housing defines a closed void substantially surrounding the shaft. A quantity of material having selected heat transfer and heat absorptive qualities is captively disposed within the closed void for controlling the temperature of both the shaft and housing bearings following engine shutdown.
More particularly, the housing defines a tortuous singular path by which heat may be conductively transferred from the turbine section of the turbocharger to the shaft bearing, or portion thereof, disposed closest to the turbine section. Consequently, substantially all conductively transferred heat reaching the turbine end bearing via the material of the housing must initially axially bypass the turbine end bearing, and then be conducted radially inwardly and axially toward the bearing in a direction toward the turbine section. The quantity of selected material in the housing void is disposed in the singular heat transfer path in heat transfer parallel with the housing material local to the bearing. The selected material is highly absorptive of heat energy at temperatures above the normal operating temperature of the turbocharger.
Turbochargers in general are well known in the pertinent art for supplying pressurized combustion air to an internal combustion Otto or Diesel cycle engine. Historically, turbochargers have been used on large engines for stationary or heavy automotive agricultural or construction vehicle applications. These turbochargers generally include a housing including a turbine housing section for directing exhaust gasses from an exhaust inlet to an exhaust outlet across a rotatable turbine. The turbine rotor drives a shaft journaled in the housing. A compressor rotor is driven by the shaft and is spaced from the turbine housing section. A compressor housing section receives the compressor rotor and defines an air inlet for inducting ambient air and an air outlet for delivering the pressurized air to an inlet manifold of the engine.
Because these past turbocharger applications involved relatively low specific engine power outputs with relatively low exhaust gas temperatures and infrequent engine shutdowns no special precautions were necessary to cool the shaft and the bearings journaling the shaft. Experience showed that the usual engine pressure oil flow lubrication which was necessary during turbocharger operation also by its cooling effect maintained the shaft and bearings at a temperature low enough to prevent oil coking in the turbocharger after engine shutdown. Because the operating temperature of the hot turbine end of the turbocharger was low enough and the mass of the turbocharger relatively large, the highest temerature experienced at the shaft and bearings after the oil flow was stopped was not high enough to degrade or coke the oil remaining in the turbocharger after engine shutdown.
However, passenger car automotive turbocharger applications have brought to light many problems. The specific engine outputs are usually higher leading to higher exhaust gas temperatures. The turbocharger itself is considerably smaller than its heavy equipment predecessor so that a smaller thermal mass is available to dissipate residual heat from the turbine housing section and turbine after engine shutdown. The result has been that heat soaking from the turbine housing section and turbine into the shaft and remainder of the turbocharger housing raise the temperature high enough to degrade or coke the remaining oil in the housing after engine shutdown. Of course, this coked oil may plug the bearings so that subsequent oil flow lubrication and cooling is inhibited. This process soon leads to bearing failure in the turbocharger.
An interim and incomplete solution to the above problem was provided by the inclusion of a hydraulic accumulator with a check and metering valve in the oil supply conduit between the engine and turbocharger. During engine operation this accumulator filled with pressurized oil. Upon engine shutdown the oil was allowed to flow only to the turbocharger at a controlled rate to provide bearing and shaft cooling while the remainder of the turbocharger cooled down. However, automotive passenger vehicles allow only sufficient space for an accumulator which is of insufficient size to dissipate the residual heat of conventional turbochargers. Under these conditions failure of the turbocharger may be accelerated.
Another more recent and more successful solution to the above problem has been the provision of a liquid cooling jacket in a part of the turbocharger housing adjacent to the turbine housing section. Liquid engine coolant is circulated through the jacket during engine operation by the cooling system of the engine so that the turbocharger temperature is relatively low. Additionally, following engine shutdown the coolant remaining in the jacket provides a heat sink so that residual heat from the turbine housing section does not increase the shaft and bearing temperatures to undesirably high levels. U.S. Pat. No. 4,068,612 to E. R. Meiners, and U.S. Pat. No. Re. 30,333 of P. B. Gordon, Jr. et al, illustrate examples of this conventional solution to the problem.
However, this latter class of turbochargers all require that engine coolant be piped to and from the turbocharger. This is usually accomplished with flexible hoses which complicate and increase the cost of the original installation of the turbocharger. Also, such plumbing requires additional maintenance and may be subject to coolant leakage which could disable the vehicle.